This invention relates to methods and devices for measuring the impedance of batteries and battery components.
Secondary batteries in stationary applications are used at electric generating stations, substations, and industrial plants to perform the vital functions of circuit breaker tripping and automatic switching, to provide for the orderly shutdown of generating units in an emergency, including the starting of emergency diesel-generators, and to power other similar tasks. Applications requiring a longer-term delivery of battery energy include emergency lighting for plants and hospitals, and supplying uninterrupted power for computer and communication equipment. Batteries must be ready to deliver their stored energy on demand to accomplish these tasks. Although batteries are reliable, a failure to supply the required energy would often result in serious consequences.
Battery systems used for these applications are often made up of individual, series-connected cells; electric utility installations generally consist of 24, 60, or 120 individual 2-volt cells connected together to provide 48, 120, or 240 VDC. The normal, steady-state load of sensors, indicating lights, relay coils, and electronic apparatus is supplied by a charger connected to the battery terminals. The charger also maintains the battery in a fully-charged state (i.e., electrolyte specific gravity of 1.230 for a typical lead-acid battery). This is known as float-charge operation. The normal load supplied by these batteries is complex, and varies with time. It is often not possible to disconnect these loads for test purposes without interrupting the main circuits they control.
The battery terminal voltage or the specific gravity of each cell making up a battery are indicators which have been used to determine battery state-of-charge. It is standard industry practice to take periodic specific gravity measurements and to conduct visual and other checks. However, specific gravity readings do not entirely indicate a battery's ability to supply power. For example, the specific gravity of each cell in a battery may indicate a fully-charged ready state, but a high impedance in one intercell connection can prevent the battery from functioning as intended.
Load discharge tests can be used to prove a battery's ability to perform. A discharge test is specified by the Nuclear Regulatory Commission (NRC) Regulations 0123 (BWR) and 0452 (PWR) for each critical battery of a nuclear generating unit. The test must be performed each time a unit is refueled. Each discharge reduces the remaining service life of a battery and may cause deterioration which can ipair the battery's ability to function next time in an on-line situation. A load discharge test may indicate trouble, but it will often not locate it.
It is important to have an in-service indication that a battery is able to deliver its stored electrochemical energy when required, and also a means for locating an abnormal condition while the battery remains in service. Routine visual inspections, specific gravity readings, voltage readings, and periodic equalizing charge are all used to keep the battery ready to perform. Measurement and testing devices have been developed for relatively simple systems which give indications of a battery's ability to deliver its stored energy.
One example described in U.S. Pat. No. 3,873,911 to Champlin utilizes an AC method at 100 Hz and low current (milliamperes) to obtain a measurement of what the patent defines as dynamic resistance. Dynamic resistance as defined by Champlin is inversely proportional to the dynamic power--an indicator of the battery's condition and state-of-charge. The disclosed method is primarily designed for testing a single automotive battery. In the disclosed Champlin system the battery to be tested is isolated from any external electrical load since a determination of the dynamic resistance/power is dependent on a measurement made using the battery's own open-circuit voltage. For this reason, any significant load on the battery during the test can distort the measurement.
U.S. Pat. No. 3,676,770 to Sharaf describes a system for indicating the charge remaining in a battery, specifically a battery for a fork lift truck, while the battery is supplying a load. For the first 75% of the battery's energy discharge, the charge level is indicated by an expanded-scale voltmeter. This measurement is used due to low initial values of battery resistance. For the remaining 25% of the charge, the battery's internal resistance increases sufficiently so that it can be compared to a reference resistance. The measurement system uses short-duration pulses of voltage or current spaced seconds apart and is powered by the battery. Typical stationary batteries on float-charge service are characterized by lower resistance and larger capacitance than those typically encountered in fork lift truck batteries. These differences reduce the reliability of measurements made in the manner described by Sharaf. Also, the charging system normally used in stationary applications produces a ripple current that contains harmonics which interact with the additional capacitance of connected cables and filters. Such additional capacitance can distort pulses, introduce spurious signals and noise, and interfere with the use of pulse timing to measure low values of battery resistance.
Another example is described by U.S. Pat. No. 3,753,094 to Furuishi which utilizes a 100 Hz AC signal at low current to measure battery internal impedance. The Furuishi patent, like the Champlin patent, provides no suggestion as to how to measure the resistance of a battery which is simultaneously connected to an active load and a battery charger. Rather, Furuishi discloses the use of blocking capacitors to prevent a flow of direct current from the battery. The object of the Furuishi device is to provide a means for checking the impedance and hence quality of a battery during the manufacturing process.
Any system that measures battery impedance must contend with the cell-to-cell variations of this characteristic. Each individual cell in a battery has a unique electrical impedance caused by manufacturing variations. For critical applications, individual cell impedances are measured when manufactured and those which closely match are selected for a battery. Normally, individual cell impedances fall within a relatively narrow range of values. This variation in individual cell impedance makes it desirable to secure a "signature" initial value of cell impedance and later to use this value to detect changes. The impedance of the entire battery varies with the sum of individual cell impedances and can be treated in a similar manner. During the service life of an individual cell, its impedance may increase, but the overall change in battery impedance may be quite small. Impedance changes may be due to reduction in charge state or may indicate a developing defect which could impair battery performance.
In measuring the impedance of a stationary secondary battery, the measuring system must contend with large capacitance values. In their September, 1959 IEEE paper entitled "Battery Impedance: Farads, Milliohms, Microhenrys", Willinganz and Rohner report that the apparent capacitance values range from 1 farad to several hundred farads, depending on battery size and test frequency. The resistance will be 0.1 to 10 milliohms, while the inductance will be less than 0.1 microhenry. It is important to consider the magnitude of these quantities when using impedance changes to determine battery condition.
It is therefore an object of the present invention to detect changes in the overall impedance of a multicell secondary storage battery, including the intercell connectors, by the continuous measurement and monitoring of the terminal to terminal impedance while the battery is floatcharged and connected to an active load.
It is also an object of the present invention to locate a defect in an individual cell of a multicell secondary storage battery, including during periods when the battery is on float charge and connected to an active electrical load.
It is another object of the present invention to locate electrical connections external to the individual cells that have impedances much greater than the statistical norm, including during periods when the battery is on float charge and connected to an active electrical load.
It is still a further object of the invention to provide a test apparatus and procedure for determining and predicting a secondary storage battery's ability to supply power to a load, including during test periods when the storage battery is on float charge and connected to an active load.